The present invention generally relates to printer apparatus and methods and more particularly relates to a printer and method adapted to reduce variability in ejected ink drop volume caused by variability in physical characteristics of ink nozzles belonging to the printer.
An ink jet printer produces images on a receiver medium by ejecting ink droplets onto the receiver medium in an image-wise fashion. The advantages of non-impact, low-noise, low energy use, and low cost operation in addition to the capability of the printer to print on plain paper are largely responsible for the wide acceptance of ink jet printers in the marketplace.
However, ink jet printers can produce undesirable image defects in the printed image. One such image defect is non-uniform print density, such as xe2x80x9cbandingxe2x80x9d and xe2x80x9cstreakingxe2x80x9d. xe2x80x9cBandingxe2x80x9d and xe2x80x9cstreakingxe2x80x9d are caused by variabilities in volumes of the ink droplets ejected from different ink nozzles. Such variabilities in ink volume can be in turn caused by variabilities between nozzles. That is, variabilities between nozzles may be caused by variability in the physical characteristics (e.g. the nozzle diameter, the channel width or length, etc.) or the electrical characteristics (e.g., thermal or mechanical activation power, etc.) of the nozzles. These variabilities are often introduced during print head manufacture and assembly.
Hence, ink dots placed by individual nozzles in the print head can be different, producing variation between rows of ink dots printed on the receiver medium. These differences are evinced by variations in the density, size, or morphology of the ink dots placed on the receiver medium by the nozzles. These variations can have a static (i.e., consistent) component and a random (i.e., non-consistent) component. Random variations between ink dots are generally less visible because their effects tend to cancel-out each other. However, static variations between ink dots placed by individual nozzles are usually more visible, thus producing xe2x80x9cbandingxe2x80x9d and xe2x80x9cstreakingxe2x80x9d image defects, which is undesirable.
An xe2x80x9caveragingxe2x80x9d technique is often used to reduce visual effects of the ink drop volume variability. That is, each image row in the printed image on the ink receiver is printed by different ink nozzles in separate printing passes. If ink volume variability between the ink nozzles is not correlated, their visual effects tend to cancel each other and are therefore reduced. The disadvantages of this technique are first, the productivity is reduced by increasing the number of printing passes; and second, although this technique can decrease the amplitude and increase the frequency of the banding artifacts, this technique cannot completely eliminate the visual effects of ink volume variation.
Techniques addressing non-uniform print density in ink jet printing are known. These techniques are generally directed to xe2x80x9cbi-modalxe2x80x9d ink jet printing apparatus wherein an ink nozzle can eject either no ink droplet or an ink droplet of a fixed volume. One such technique relies on the control of ink volume by changing the activation voltage width of an electric signal supplied to the print head for activating ink ejection. This technique is limited in that bubble formation occurs to cause ink droplet ejection, which bubble formation complicates formation of desired ink drop volume.
Another bi-modal ink jet printing technique is disclosed in U.S. Pat. No. 5,038,208 titled xe2x80x9cImage Forming Apparatus With A Function For Correcting Recording Density Unevennessxe2x80x9d issued Aug. 6, 1991 in the name of Hiroyuki Ichikawa, et al. This technique relies on varying a halftoning pattern of a group of ink dots in the printed image in order to correct ink drop volume variations. This second technique is, however, time consuming because it requires intense image processing for each input image file. In addition, this technique is also limited to bi-modal ink jet printing.
Therefore, there has been a long-felt need to provide a printer and method adapted to reduce variability in ejected ink drop volume caused by variability in physical characteristics of ink nozzles belonging to the printer.
An object of the present invention is to provide a printer capable of reducing printing variabilities between ink nozzles during multiple tone ink jet printing.
With this object in view, the present invention resides in a printer, comprising a print head having a plurality of ink chambers therein capable of ejecting a plurality of ink droplets therefrom, at least one of the chambers having undesirable characteristics that produce ink droplets of non-nominal volume; and a controller connected to said ink chambers for transmitting electronic waveforms to the ink chambers to eject the ink droplets so as to compensate for non-nominal volume, so that at least one chamber produces ink droplets of nominal volume.
According to one embodiment of the present invention, a print head includes a plurality of nozzles integrally attached to the print head, each nozzle being capable of ejecting an ink droplet therefrom. At least one of the nozzles has undesirable characteristics producing ink droplets of non-nominal volume. A waveform generator is connected to the nozzles for generating a plurality of waveforms to be supplied to the nozzles, so that the nozzles eject the ink droplets in response to the waveforms supplied thereto. Each waveform has a plurality of pulses controlling ink droplet volume. A look-up table is connected to the nozzles for storing values of number of pulses to be supplied to each nozzle. A calibrator in communication with the look-up table is provided for converting the pixel values describing each pixel of an input image file to a calibrated image file. An image halftoning unit is connected to the image calibrator for halftoning the calibrated image file to generate a halftoned image file having a plurality of pixel values. A nozzle selector interconnects the waveform generator and the print head for selecting predetermined ones of the nozzles for activation. Moreover, a controller is connected to the image halftoning unit and the nozzle selector for controlling the waveforms supplied to the nozzles in order to eject the ink droplets in a manner that compensates for the non-nominal volume by controlling the number of pulses in the waveform. In this manner, at least one chamber produces ink droplets of nominal volume. In addition, a piezoelectric electromechanical transducer is disposed in at least one of the nozzles, the transducer being responsive to the waveforms for ejecting the ink droplet from the at least one nozzle.
An advantage of the present invention is that ink drop volume variability between nozzles is compensated, so that ink drops ejected from each ink nozzle is substantially identical regardless of the physical or electrical characteristics of the nozzle.
Another advantage of the present invention is that calculation for compensating ink drop volume variation between nozzles can be made efficiently using a look-up table.
A further advantage of the present invention is that quality images nonetheless can be produced by ink jet print heads having manufacturing variabilities. In other words, robustness of a multiple-tone inkjet printer is improved.
These and other objects, features and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described illustrative embodiments of the invention.